Treatment for well-differentiated thyroid cancer (WDTC) includes thyroid surgery often followed by radioactive iodine (RAI) and suppressive doses of thyroid hormone replacement. The value of routine use of RAI is, however, passionately debated. Reflecting this controversy, our preliminary studies show tremendous variation in RAI use between 1,423 hospitals which encompasses over 200,000 patients. This variation in RAI use has health and financial implications, but the correlates of use remain unknown. Through both multilevel hierarchical modeling of retrospective cohort data including over 280,000 WDTC patients over a 21 year period and a two-sample comparison of data acquired from 1575 physician surveys, we will address the relationship between characteristics of the patient, provider, and hospital relative to RAI use as treatment for WDTC. We expect to find unwarranted variation in RAI use with variables other than disease severity predicting administration. Based on studies in other malignancies, we anticipate that provider knowledge, attitudes, and beliefs will be key determinants of RAI use. Not only will the proposed project impact the current management of thyroid cancer, it will also serve as a vehicle for the applicant's development into an independent researcher in health services research. The applicant already has a strong record of research productivity and a clinical foundation in thyroid cancer. Through select coursework, mentorship from national experts in health services research, and completion of a carefully designed project linking skill acquisition to research aims, the applicant will emerge as an expert in health services research as it pertains to thyroid cancer and will be well-positioned for an independent research career in endocrine oncology.